In 1943, an alternative drive system for the Panzer IV entered development. This was the Hydrostatischem Antrieb or Hydrostatic Drive, also known as the “Thoma” drive.
It was designed and produced in the Augsburg plant of ZF Friedrichshafen, and tested on a turret-less Panzer IV Ausf.G chassis that had been badly damaged during combat operations.
The Thoma system operated in a similar way to the petrol/electric drive system produced by Porsche for his Tiger I concept vehicle that would later become the Ferdinand/Elefant. This system was a lot safer, however, as it was a petrol/hydraulic system. This gave the benefits of the Porsche system without the fire risk that plagued it so badly.
The Panzer IV chassis underwent heavy modifications to be able to mount this new drive system. The engine compartment of the tank was almost completely removed and rebuilt. The drive was placed in the rear of the tank under a large sloping engine deck. Two oil pumps were installed behind, and connected directly to the normal Maybach HL 120 TRM engine. These powered two hydraulic motors. A swash plate drive sent the power through a reduction gear into the newly added rear drive wheels, which replaced the traditional idler wheel.
Inside the crew compartment, the old drive shafts were removed along with the large gearbox and final drive assembly at the bow end of the vehicle. The traditional steering tillers were replaced with a crescent-like wheel, similar to the one found on Tiger I. Directional movement was achieved by two control cylinders. These cylinders regulated the volume of the oil inside the pump. This governed the amount of power the drive wheels would receive. Two large 780mm adjustable toothed idlers replaced the original Panzer IV drive sprockets.
Later in 1944, the vehicle was tested with a hydraulically powered turret. Unfortunately, more information on this modification is unavailable.
Only one prototype of the vehicle with this drive system was built by the time the Allies were knocking on Germany’s door. In April 1945, the US 3rd Infantry Division was advancing through southern Germany and into Bavaria. They broke into Augsburg on the 27th and had the whole city secured by the 28th. With the city, they captured the Zahnradfabrik plant, and the test vehicle.
After the war, the vehicle was shipped back to the United States, where it was subjected to thorough tests by Vickers Inc. Detroit, Michigan until at least 12th April 1946, when a report stating how the drive worked was drafted:
“The powertrain consisted of two staggered-plate oils pumps that are assembled as a unit and are driven by a 12-cylinder Maybach engine. Oil is pushed by the pumps to two separate oils engines which power the drive wheels of the tracks. The oil engines are attached to the final drive housings. The engine and power aggregate are located in the rear of the vehicle, and the vehicle is moved by rear mounted drive wheels. The volume of the pumps is controlled by the driver, who thereby controls the torque of the various pressure conditions that are created by the steering and stopping of the vehicle. In the same manner, the forward and backward movement of the vehicle is achieved by directing oil flow. Pressurized oil to activate the pumps and engines and for the high-pressure connections was advanced by a geared-wheel pump that was connected to the vehicle’s engine by direct drive.”
Unfortunately, the German test data has been lost to history. The vehicle was left in the open, exposed to the elements, at the U.S Army Ordnance Proving Grounds, Aberdeen in Maryland. In 2015 it was moved to the U.S. Army Center for Military History Storage Facility, Anniston, AL, USA, where it has the officially long-winded designation of “Tank, Medium, Full Track, Experimental Transmission, German Army, Steel, Tan, PzKpfw IV, 75mm Gun, German, 1945, World War II”.
An article by Mark Nash
Panzer IV mit Hydrostatischem Antrieb
|Dimensions||5.41 x 2.88 x 2.68 m (17.7×9.4×8.8 ft)|
|Total weight, battle ready||25 tons|
|Armament||Rheinmetall 75 mm (2.95 in) KwK 40
2-3 MG 34/MG 42 7.92 mm (0.31 in) machine-guns
|Armor||From 15 to 65 mm (0.59-2.56 in)|
|Propulsion||Maybach V12 gasoline HL 120 TRM
(220 kW) 300 bhp@2500 rpm
|Speed on /off road||42 km/h (26 mph)|
Links & Resources
Panzer IV und seine Varianten (Panzer IV and its Variants) Spielberger and Doyle.
Panzer Tracts No. 4, Panzerkampfwagen IV, Grosstraktor to Panzerbefehlswagen IV
Panzer Tracts No. 4-3, Panzerkampfwagen IV Ausf.H and Ausf.J, 1943 to 1945
German Tanks of ww2